1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which uses a magneto-resistive effect element as a storage element.
2. Description of the Related Art
In recent years, research and development of a magnetic random access memory which stores data in a non-volatile manner by utilizing a magneto-resistive effect have been frequently carried out.
The magnetic random access memory stores data “1” and “0” by using a magneto-resistive effect element (a TMR element, a GMR element or the like). The basic structure of the magneto-resistive effect element is a structure sandwiching a tunneling barrier by two magnetic layers.
Data stored in the magneto-resistive effect element is judged by whether directions of magnetization of the two magnetic layers are the same (parallel state) or opposite to each other (anti-parallel state).
Here, an anti-magnetic layer for fixing a direction of magnetization of a fixed layer is brought into contact with one (fixed layer) of the two magnetic layers. As a result, data to be stored in the magneto-resistive effect element is actually determined based on a direction of magnetization of the other one (free layer) of the two magnetic layers.
When magnetization of the magneto-resistive effect element enters the parallel state, the tunnel resistance of the tunneling barrier sandwiched between the two magnetic layers constituting the magneto-resistive effect element becomes lowest. This state is, for example, the state “1”. Further, when magnetization of the magneto-resistive effect element enters the anti-parallel state, the tunnel resistance of tunneling barrier sandwiched between the two magnetic layers constituting the magneto-resistive effect element becomes highest. This state is, for example, the state “0”.
In the magnetic random access memory, a write current is used when programming data. The write current is caused to flow through two write lines crossing each other at a point where the magneto-resistive effect element is positioned. The state of magnetization of the magneto-resistive effect element is controlled by a combined magnetic field generated by the write current flowing through the two write lines.
Meanwhile, one of characteristics of the magnetic random access memory lies in higher integration of a memory cell (magneto-resistive effect element). However, when realization of the finer element and higher integration advances, irregularities in the element characteristic caused due to shifting of a pattern, irregularities in an element shape or the like in manufacture can not be ignored.
For example, when there occur irregularities in a threshold voltage of a transistor constituting a write driver due to irregularities in process, a large write current which generates a magnetic field exceeding the intensity of the magnetic field required for programming may be possibly caused to flow through the write lines.
In this case, program data may be erroneously written in a non-selected memory cell (magneto-resistive effect element) adjacent to a selected memory cell (magneto-resistive effect element) which is a target of programming.